Observations of Polar Mesospheric Clouds Compared with MLS Temperatures and Water Vapor Measurements

Polar Mesospheric Clouds (PMCs), which are also known as noctilucent clouds, occur poleward of 50° at altitudes of 80 to 86 km during a 3 month period centered 2 to 3 weeks after summer solstice in each hemisphere. This is when the upper mesosphere gets cold enough for the small amounts of water vapor in the atmosphere to freeze forming ice particles. The Solar Backscatter Ultraviolet [SBUV/2] instrument on NOAA-18 and the Microwave Limb Sounder [MLS] on Aura, have nearly the same equator crossing time ~1345. So despite different orbital periods (102 minutes versus 99 minutes), there will be frequent coincident measurements, which will allow us to compare the detections of PMC by the SBUV/2 instrument with the coincident temperature and water vapor measurements by MLS. For a coincidence we require the tangent point for the MLS measurement to be inside the SBUV/2 170x170 km field of view, within 15 minutes. This provides several thousand coincidences during each PMC season (30 days prior to Summer solstice to 70 days after). There are four PMC seasons available for comparison in the Northern Hemisphere starting with May 2005 launch of NOAA-18, and five PMC seasons to date in the Southern Hemisphere. As expected, for the events where SBUV detected a PMC, the air was colder than those events where no PMC was detected. Nearly all the PMC events occurred when the temperature was below 150 K, and nearly half of the non-PMC events corresponded to temperature greater than 150 K. We calculated the saturation ratio with respect to ice from the MLS measurements of temperature and water vapor. The PMC events occurred at higher saturation ratios than the non-PMC events, with nearly all of them occurring with saturation ratios greater than 1.0. That PMCs were detected with saturation ratios less than 1.0 could be due to uncertainties in the PMC altitude since the nadir viewing SBUV provides no vertical information on the PMC or the limited vertical resolution (12 to 14 km) of the MLS measurements compared with the 2 to 3 km thickness of the PMCs.